U.S. patent number 5,151,588 [Application Number 07/807,632] was granted by the patent office on 1992-09-29 for radiation imaging apparatus having detection elements of varying sizes.
This patent grant is currently assigned to Shimadzu Corporation. Invention is credited to Susumu Adachi, Motosada Kiri.
United States Patent |
5,151,588 |
Kiri , et al. |
September 29, 1992 |
Radiation imaging apparatus having detection elements of varying
sizes
Abstract
A radiation imaging apparatus provided with a detector array
comprising a plurality of detector units, each of which, in turn,
comprises a plurality of X-ray detecting elements each having a
photosensitive surface of a different area from those of the other
elements and corresponds to one picture element or pixel of a
display, the output signal from one or more of the X-ray detecting
elements of each of the detector units being used as image
information for a corresponding one of the pixels of the display.
While the X-ray dose remains at a low level, the detection output
from the detecting element having a large photosensitive surface or
the sum of the output from that element and the outputs from the
other elements having smaller photosensitive surfaces is used as
pixel information to form an image of the object being examined.
When the radiation dose increases to a predetermined high level,
the result of detection by only the element having a small
photosensitive surface is used as pixel information.
Inventors: |
Kiri; Motosada (Kyoto,
JP), Adachi; Susumu (Osaka, JP) |
Assignee: |
Shimadzu Corporation (Kyoto,
JP)
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Family
ID: |
17947640 |
Appl.
No.: |
07/807,632 |
Filed: |
December 13, 1991 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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612958 |
Nov 15, 1990 |
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Foreign Application Priority Data
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Nov 24, 1989 [JP] |
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1-305646 |
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Current U.S.
Class: |
250/208.1;
348/E5.086; 250/370.09; 348/164 |
Current CPC
Class: |
G01T
1/2928 (20130101); G01N 23/04 (20130101); H05G
1/64 (20130101); H05G 1/26 (20130101); H04N
5/32 (20130101) |
Current International
Class: |
G01T
1/29 (20060101); G01T 1/00 (20060101); G01N
23/04 (20060101); G01N 23/02 (20060101); H05G
1/00 (20060101); H05G 1/26 (20060101); H05G
1/64 (20060101); H04N 5/32 (20060101); H01J
040/14 () |
Field of
Search: |
;250/208.1
;358/213.15,213.22,213.27 ;357/3D |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0137487 |
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Apr 1985 |
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EP |
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0217456 |
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Apr 1987 |
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EP |
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0287197 |
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Oct 1988 |
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EP |
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0362427 |
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Apr 1990 |
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EP |
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2613831 |
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Oct 1988 |
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FR |
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Primary Examiner: Nelms; David C.
Assistant Examiner: Shami; K.
Attorney, Agent or Firm: Wegner, Cantor, Mueller &
Player
Parent Case Text
This application is a continuation of U.S. application Ser. No.
07/612,958, filed Nov. 15, 1990, now abandoned.
Claims
What we claim is:
1. A radiation imaging apparatus comprising:
means for exposing an object to be examined to electromagnetic
radiation;
a detector array comprising a plurality of radiation detector units
each of which corresponds to one of the pixels of a display and
comprises a plurality of radiation detecting elements for detecting
the radiation from said object under examination, each of said
detecting elements having a photosensitive surface of a different
area from those of the other detecting elements;
a signal processing circuit of a photon counting type for
processing the output signals of said detecting elements to produce
an output corresponding to the number of the photons detected by
each of said detecting elements; and
a data processor for selecting one or more of said detecting
elements of each of said detector units in accordance with the
radiation dose received by said detecting elements so that the
output of said selected one or more detecting elements is used as
pixel information to form a radiation image of said object on said
display.
2. The apparatus of claim 1, wherein said signal processing circuit
comprises a plurality of amplifiers each connected to one of said
detecting elements to produce an output pulse in accordance with
the radiation dose received by said detecting elements, and a
plurality of pulse counters each connected to one of said
amplifiers to count said output pulses from each of said amplifiers
so as to produce an output corresponding to the number of counts of
said pulses.
3. The apparatus of claim 1, wherein said data processor operates
in such a manner that while the radiation dose received by said
detecting elements remains at a low level, either the output from
that one of said detecting elements of each of said detector units
which has a large photosensitive surface, or the sum of the output
from said one detecting element and the outputs from said other
detecting elements which have smaller photosensitive surfaces is
used as pixel information to be supplied to said display to form a
radiation image of said object, and that when said radiation dose
increases to a predetermined high level, the output from that one
of said detecting elements of each of said detector units which has
a small photosensitive surface is used as pixel information to be
supplied to said display to form a radiation image of said object.
Description
BACKGROUND OF THE INVENTION
This invention relates to an apparatus for producing an X-ray image
by using an array of X-ray detecting elements, such as an X-ray
computed tomograph commonly referred to as an X-ray CT for medical
use or non-destructive testing apparatus for industrial use.
The X-ray imaging apparatus is provided with a detector array
composed of a plurality of X-ray detecting elements for detecting
X-rays from an object being examined. For collection of data to
obtain an X-ray image of the object, it has been customary to
conduct an analog conversion of the current output from each of the
radiation detecting elements to corresponding voltage signals. In
recent years, what is called the photon counting system has been
proposed, in which the pulse-like signals produced by each of the
X-ray detecting elements are counted to obtain necessary image
information.
The analog current-voltage conversion system has a disadvantage
that the dynamic range of the system is so low that satisfactory
images cannot be obtained. In particular, as the level of the
output signal from the detecting elements lowers, the signal to
noise ratio decreases because of remaining constant amplifier
noise, so that there is a limit to detection of the signal.
The photon counting system has no such limit in a low range of
radiation dose. As the radiation dose increases, however, the X-ray
photons from the object under examination are more likely to be
miscounted, so that there is a limit to detection of the radiation
at a higher level of radiation dose.
After all, neither of the two systems is able to provide a
sufficient performance to produce quality images.
Accordingly, the primary object of the invention is to eliminate
the above disadvantages of the prior art systems and to provide an
X-ray imaging apparatus which has a wider dynamic range than the
prior art systems.
The invention will be described in detail with reference to the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of one embodiment of the
invention;
FIG. 2 is a graph showing the relation between the number of
incident X-ray photons on the X-ray detecting elements provided in
the apparatus of FIG. 1 and the number of X-ray photons actually
counted; and
FIG. 3 shows the shapes of the detecting elements and a layout of
the detector array composed of the elements in another embodiment
of the invention.
SUMMARY OF THE INVENTION
In the X-ray imaging apparatus of the invention, the output signals
from a detector array is applied to a signal processing circuit of
the photon counting type so as to determine the X-ray radiation
dose, and the result of the determination is used as pixel
information to form an X-ray image of the object being examined.
The detector array comprises a plurality of detector units, each of
which, in turn, comprises a plurality of X-ray detecting elements
each having a photosensitve surface of a different area from those
of the other elements. Each of the detector units corresponds to
one picture element or pixel of a display unit, and the output
signal from one or more of the X-ray detecting elements of each of
the detector units is used as image information for a corresponding
one of the pixels of the display.
The X-rays from an object being examined enter the detecting
elements of each of the detector units. The detecting element
having a large photosensitive surface receives a higher radiation
dose than the detecting elements having smaller photosensitive
surface areas. As the radiation dose increases, errors are likely
to occur to the result of detection by the detecting element having
a large photosensitive surface due to miscounting. With the
detecting elements having smaller photosensitive surfaces, however,
accurate detection is ensured up to a higher level of radiation
dose.
In accordance with the invention, while the X-ray dose remains at a
low level, the detection output from the detecting element having a
large photosensitive surface or the sum of the output from that
element and the outputs from the other elements having smaller
photosensitive surfaces is used as pixel information to form an
image of the object being observed. When the dose increases to a
predetermined high level, the result of detection by only the
element having a small photosensitive surface is used as pixel
information, thereby to extend the limit of detection to a higher
level of radiation dose than in the photon counting system.
The invention will be described in detail with reference to the
accompanying drawings.
DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 1 shows one embodiment of the invention, wherein there is
shown a detector array 10 which comprises a plurality of detector
units 11.sub.1, 11.sub.2, . . . 11.sub.n arranged
two-dimensionally, that is, on a plane. Each of the detector units
11.sub.1, . . . 11.sub.n comprises a plurality, say, three
detecting elements Sa, Sb and Sc each having a photosensitive
surface of a different area from those of the other elements.
The output signals from the detecting elements Sa, Sb and Sc of
each of the detector units 11.sub.1, . . . 11.sub.n are applied to
a signal processing unit 20 of a photon counting type. In
particular, the unit 20 comprises a plurality of amplifiers such as
charge sensitive amplifiers 21.sub.1a, 21.sub.1b, 21.sub.1c,
21.sub.2a, 21.sub.2b, 21.sub.2c, . . . 21.sub.na, 21.sub.nb,
21.sub.nc, and a plurality of corresponding counters 22.sub.1a,
22.sub.1b, 22.sub.1c, 22.sub.2a, 21.sub.2b, 22.sub.2c, . . .
22.sub.na, 22.sub.nb, 22.sub.nc connected to the outputs of the
amplifiers 21.sub.1a, . . . 21.sub.nc, respectively. The output
from each of the detecting elements Sa, Sb and Sc of each of the
detector units 11.sub.1, . . . 11.sub.n is amplified by the
corresponding one of the amplifiers 21.sub.1a, . . . 21.sub.nc to a
logic level so as to become a pulse-like voltage signal, which is
applied to the corresponding one of the counters 22.sub.1a, . . .
21.sub.nc. In this manner, the numbers of X-ray photons incident on
the detecting elements are counted by the respective counters.
The outputs from the counters are applied to a data processor 30,
which provides necessary data for forming an X-ray image of the
object under examination. The data processor 30 is so designed
that, with each of the detector units 11.sub.1, . . . 11.sub.n
corresponding to one of the pixels of a display unit on which the
image of an object under examination is to be displayed, the
processor operates in accordance with an algorithm to be described
hereinafter to automatically determine which of the detecting
elements Sa, Sb and Sc of each of the detector units is to be
selected to obtain pixel information from its output, and to form
an image in accordance with the pixel information obtained from the
selected detecting element.
As previously mentioned, with a given fixed amount of radiation
incident on the detector array, as the area of a detecting element
increases, the number of X-ray photons incident on the element
increases so that miscounting of photons is more likely to occur.
FIG. 2 shows a graph showing the relation between the number of
photons incident on each of the detecting elements Sa, Sb and Sc
having different photosensitive surface areas and the number of
photons actually counted, with the number of photons counted taken
along the ordinate and the number of photons incident on the
detecting elements taken along the abscissa. The curves designated
by Sa, Sb and Sc result from the corresponding detecting elements
Sa, Sb and Sc.
As shown in the graph, the detecting element Sa having a large
photosensitive surface detects photons with a high sensitivity in
the range of low radiation dose, with the line Sa in the graph
being linear in the low range. As the radiation dose increases,
however, due to miscounting the linear relation between the number
of X-ray photons that hit the detecting element having a large
photosensitive surface and the number of X-ray photons that are
actually counted comes to be lost. In other words, the latter
number will not be proportional to the former number in a range of
higher radiation dose.
On the other hand, although the detecting element Sb or Sc having a
smaller photosensitive surface has a lower sensitivity than the
detecting element Sa in the range of low radiation dose, the
above-mentioned linearity is maintained in the range of higher
radiation dose.
Therefore, if the number of counts of the photons from only the
detecting element Sa having a large photosensitive surface, or the
sum of the numbers of counts of the photons from the elements Sa
and Sb, or Sa, Sb and Sc is used as image information for the pixel
corresponding to the detector unit when the radiation dose incident
thereon is in a low range, and the number of counts of the photons
from the detecting element Sb or Sc is used when the radiation dose
is in a high range, it is possible even with the photon counting
system to provide an X-ray image with a high degree of accuracy in
a range of high as well as low radiation dose.
Suppose that the relation between the number of the photons
incident on each of the detecting elements and those of the photons
actually counted is as shown in FIG. 2. While the number of
incident photons is bellow n.sub.1, the number of counts by the
element Sa is used. When the number of incident photons is between
n.sub.1 and n.sub.2, the sum of the numbers of counts by both the
elements Sa and Sb is used. When the number of incident photons is
between n.sub.3 and n.sub.4, the sum of the numbers of counts by
both the elements Sb and Sc is used. Finally, when the number of
incident photons exceeds n.sub.4, the number of counts by the
element Sc only is used. In this manner it is possible to form an
accurate X-ray image from a low to a high level of radiation dose.
In this case it is necessary to correct the counts by the detecting
elements Sa, Sb and Sc in accordance with the areas of the
photosensitive surfaces of the elements.
The previously mentioned algorithm used by the data processor 30 is
such that it determines whether or not the number of photons
counted by each of the counters 22.sub.1a, . . . 22.sub.nc
connected to the detecting elements Sa, Sb and Sc of the detector
units 11.sub.1, . . . 11.sub.n exceeds a preset value and, on the
basis of the result of the determination, selects one or more of
the detecting elements Sa, Sb and Sc the number of photons counted
by which is to be used as pixel information for forming an image of
an object under examination in the previously mentioned manner.
In accordance with the invention, the data processor 30 need not
necessarily be provided with the above-mentioned function of
automatically selecting the detecting elements. The arrangement may
also be such that an operator selets one or more of the detecting
elements when a measurement is conducted.
In the illustrated embodiment, each of the detector units which
constitutes one pixel comprises three detecting elements Sa, Sb and
Sc. They may also comprise two or more detecting elements. The
detector array may comprise detecting elements having a
photosensitive surface of any other suitable shape than in the
illustrated embodiment.
FIG. 3 shows another embodiment of the invention, wherein the
detector array comprises a plurality of large detecting elements S1
having a photosensitive surface of octagonal shape and a plurality
of small detecting elements Ss having a photosensitive surface of
square shape, with four large elements being arranged adjacent the
four sides of one small square element.
The detecting elements constituting the detector array may be
arranged linearly or two-dimentionally. The array may be held
stationary or so arranged as to be able to be scanned.
As mentioned above, in the X-ray imaging apparatus of the invention
each of the detecting elements constituting a detector unit has a
photosensitive surface of a different area than the other elements,
and one or more of the detecting elements is selected in accordance
with the radiation dose they receive so that the number of counts
of photons by the selected element or elements is used as pixel
information to form an X-ray image of the object under examination.
Basically, the apparatus is a photon counting system having an
advantage that it is free of noise and can form an X-ray image with
a high degree of accuracy even in a range of low radiation dose. At
the same time, the disadvantage of the photon counting system that
there is a limit to detection in a range of high radiation dose due
to miscounting can be eliminated, so that it becomes possible to
obtain accurate X-ray images in a very wide dynamic range from a
low to a high level of radiation dose.
* * * * *